1. Field of the Invention
The present invention relates to observation optical systems such as telescopes and binoculars and, more particularly, to an observation optical system, having the zoom function of varying the magnification of an image being observed.
2. Description of Related Art
Mainstream, conventional zoom binoculars perform zooming by a variable magnification eyepiece lens having a moving lens unit. FIG. 36 is a schematic diagram showing one example of conventional zoom binoculars, in which, for the purpose of simplicity, only one of the right and left optical systems of the binocular is illustrated.
In FIG. 36, reference numeral 100 denotes an objective lens of positive refractive power. An erect prism P is provided for erecting, and laterally reversing an object image OBI formed mainly by the objective lens 100. A variable magnification eyepiece lens 200 comprises a first lens unit 201 of negative refractive power, a second lens unit 202 of positive refractive power and a third lens unit 203 of positive refractive power. The object image OBI is formed at a position before (on the object side of) the second lens unit 202 through the objective lens 100, the erect prism P and the first lens unit 201 of the eyepiece lens 200.
In the binocular of such a construction, the first lens unit 201 of the variable magnification eyepiece lens 200 is made axially movable for varying the image magnification, and to compensate for the image shift, or to correct the diopter, the second lens unit 202 is made to axially move.
Such a variable magnification eyepiece lens 200 is favorable to realize zoom binoculars relatively easily. Up to now, therefore, this arrangement has been very popularly employed.
However, the use of the above-described construction and arrangement for the variable magnification eyepiece lens in zoom binoculars results in as narrow an apparent angle of view as 35-40 degrees at the low magnifications. To widen the field coverage, there is a need to increase the diameter of the second lens unit 202 and the bulk of the erect prism P. For this reason, it is difficult for the prior art arrangement to simultaneously fulfill the requirements of increasing the apparent field angle and of minimizing the bulk and size of the binocular.